In many personal and industrial applications, safety knives are desirable to prevent user laceration both before a cut is made and immediately subsequent to a cut. These safety knives may be disposable or designed for extended use. The knives that are used for extended periods may have replaceable blades or blade cartridges so that the handle and related safety apparatus may be reused while still maintaining a sharp cutting edge. The disposable knives may have the blade permanently attached to within the safety knife, so that the knife is discarded once the blade becomes dull. For example, the disposable knife is desirable in the food services industry, where loose blades from replaceable blade knives may find their way into the food product. A permanently attached blade forces the user to discard the entire knife rather than just change the blade. To reduce replacement costs incurred by the customer, these disposable knives are often made of inexpensive plastic materials with the least possible number of injection molded parts and moving parts.
Some safety knives have blade covers that may be retracted upon actuation of a trigger or similar actuation means. When locked, these blade covers are prevented from retracting due to the blade cover movement being blocked by a pawl-like mechanism that engages a stop. Only after being disengaged from the stop, can the blade cover be retracted to expose the blade. Some of these newer safety knives further include features that permit just one blade cover retraction for each trigger pull. If the trigger is continuously pressed after the blade cover has been retracted rather than released, the blade cover will still become locked in the extended position. Only after releasing the trigger and depressing the trigger a second time will the blade cover be permitted to retract once again.
The single-use per press feature has been achieved at this point through use of complex mechanisms that require tight tolerances and close fits between parts. For example, much of the art has required the use of one-way ramp features, such that a moving part must ride up the ramp feature, thereafter falling off a ledge so that the part may not return to the previous position until a second actuation has occurred. This ramped design requires the moving part to be closely and exactly situated relative to the ramp feature. The ramped feature must be small, however, so that movement of the moving part up the ramp is not unduly hindered. Further, many current designs require flexible plastic parts, which tend to fatigue and slightly change shape, causing parts to eventually lose contact or move out of optimal alignment. Small features combined with close tolerances between moving parts create a safety mechanism that is relatively expensive to manufacture, and potentially unreliable due to part fatigue.
Since many of the safety knives are disposable or otherwise have short lives, any added expense greatly detracts from the competiveness of the knife. It is desirable to eliminate tight tolerances when the product is disposable to ultimately reduce costs and encourage timely disposal of the knife to prevent usage when dull. It is desirable to have a safety knife that is inexpensive to manufacture and more reliable due to the loosening of tolerances, reduction in parts and overall simplification of the mechanism.